Blender Sculpt brushes don't function properly
I was having this issue too. I don't know why this is occurring but I can confirm that Mr Zak's solution worked for me.  

To clarify on his instruction:


Create a backup of your work
Switch to object mode
Select only the model you want to sculpt (you can only choose one at
a time)
Press Ctrl + A
Select Scale from the pop-up menu


The fix is complete and you may now switch back to sculpting if you wish.